How long does it take to scan and IP? In general?

it is made intelligent so example if you scan a C class 255 IPs typically many of those do not really run anything or have everything blocked incoming, the it can determine this and finish scans fast example in 10-20 mins

Then if you have a server with multiple services, web , mail, SQL, AD, then it can take few hours depending what it finds.

So it will not launch checks for a service if it did not identify the service first